Comet nucleus
A comet nucleus is the solid icy body at the heart of a comet, usually a few kilometres across and very dark.
The nucleus is the solid part of a comet. It is a lump of ices mixed with dust and rock, often described as a dirty snowball or, because dust may dominate, an icy dirtball. Most nuclei are between about 1 and 20 kilometres across, though some are bigger.
Spacecraft have photographed several nuclei, starting with Halley's Comet in 1986. They are very dark, reflecting only a few per cent of sunlight, because their surfaces are coated with dust and organic material. They are also very porous: comet 67P has a density of only about 0.5 grams per cubic centimetre, less than water.
As the nucleus nears the Sun, ice turns to gas in active spots, sometimes in jets, lifting dust into the coma. Over many orbits, the nucleus may lose its ice and become dormant, looking like an asteroid. Some near-Earth asteroids may be such extinct comets.
Related terms
Level: Explorer Ages 10 and up